Conventional radius end mill includes chip ejection grooves, peripheral cutting edges, bottom cutting edges and corner cutting edges. For example, each of the chip ejection grooves is formed at a peripheral portion of an end mill body located close to a distal end of the body so as to be twisted backward in the rotation direction of the end mill as the chip ejection groove approaches the terminal end of the end mill body. Each of the peripheral cutting edges is formed at a wall surface of the chip ejection groove facing forward in the rotation direction of the end mill so as to be along a ridge line of the wall surface to be close to the outer circumference of the end mill body. Each of the bottom cutting edges is formed at the wall surface of the chip ejection groove so as to be along a ridge line of the wall surface located close to the distal end of the end mill body. Each of the corner cutting edges is formed between the peripheral cutting edge and the bottom cutting edge so as to communicate the peripheral cutting edge with the bottom cutting edge, and is formed like an arcuate line which curves outward in the radial direction of the end mill body and which curves toward the distal end of the end mill body from the terminal end thereof.
Recently, in order to improve the cutting efficiency of dies, high feed-speed cutting is generally performed. The high feed-speed cutting means that the feed speed of a cutting device such as an end mill is accelerated. The high feed-speed cutting is performed, the cutting amount in the forward direction per one cutting edge is increased. Thus an outline of the cutting edge is transcribed to a surface of a material to be cut, and thereby the surface of the material becomes rough. Further, the cutting resistance acting on the cutting edge is increased, and thereby the lacking and chipping of the cutting edge may be happened. Furthermore, if the chips become large, the chips are hardly ejected from the groove easily. Therefore, the cutting resistance is increased depending upon blocking of the chips within the ejection groove, and the surface of the material to be cut is damaged by the chips.
In order to resolve the above problems, radius end mills for the high feed-speed cutting are disclosed in Japanese Patent Publication No. 3590800 (“JP '800”) and Japanese Unexamined Patent Application, First Publication No. 2004-82275 (“JP '275”). With the radius end mill disclosed in JP '800, a curvature radius of the corner cutting edge is greater than or equal to 40% of the outer diameter of the end mill and is less than 50% thereof. In addition, the radius end mill is formed so that a center area of a distal end surface thereof subsides. The curvature radius of the corner cutting edge of this end mill is larger than that of conventional end mill Therefore, the outline of the cutting edge transcribed to the surface of the material to be cut becomes smooth. As a result, if the feed speed of the cutting device is increased, it is possible to make the surface of the material smooth.
With the radius end mill disclosed in JP '275, a rake surface of each corner cutting edge curves upwardly. In addition, a chip space recessed toward a distal end of an end mill body is formed at a part of the corner cutting edge located close to an outer circumference of the end mill body so as to communicate with the rake surface. Since the rake surface of the corner cutting edge of this end mill curves upwardly, the intensity of the corner cutting edge can be improved. Therefore, it is possible to generate lacking and chipping of the cutting edges. Accordingly, it is possible to stably-cut the material by the radius end mill In addition, since chips can be discharged through chip base continuously formed on the rake surface, the chips separate from the rake surface immediately. Accordingly discharge of the chips can be improved. Therefore, it is possible to prevent the chips from blocking the recessed chip space.
When a material is cut to produce dies or the like, as shown in FIG. 5, a pocket of which an opening is formed like a substantially multi-angle shape may be formed on the surface of the material. When the pocket is formed, at a part of an outline of the pocket having a number of corners, the outer circumference of the end mill contacts a side surface of the material in which the pocket is formed, thus the contact length of the cutting edge with respect to the side surface of the material is elongated. In particular, at an angle part of the pocket, the contact length of the cutting edge with respect to the side surface of the material is further elongated, thus a vibration of the end mill is caused by increase of the cutting resistance. Therefore, there is a problem that the smoothness of the surface of the material to be cut deteriorates.
Further, with the radius end mill disclosed in JP '800, the contact length of the corner cutting edge is elongated, since the curvature radius of the cutting edge is large. Thereby, the cutting resistance is increased. In particular, when the angle parts and the outline part of the pocket are cut, the contact length of the cutting edge is further elongated. Thus, the vibration of the end mill is caused by an increase of the cutting resistance. In order to prevent the end mill from vibrating, it is necessary to reduce the forward cutting speed of the end mill Therefore, there is a problem that the cutting efficiency of the material deteriorates.
With the radius end mill disclosed in JP '275, since the rake surface of the corner cutting edge curves upwardly, the contact length of the corner cutting edge is elongated. Thus the cutting resistance is increased. Therefore, similar to the radius end mill disclosed in JP '800, when the angle parts and the outline part of the pocket are cut, it is necessary to reduce the feed speed of in order to prevent the end mill from vibrating. Therefore, there is a problem that the cutting efficiency of the material deteriorates. In addition, with the radius end mill, the rake surface must be formed upwardly, and the chip space for discharging the chips must be formed. Thus, the shape of the cutting edges is extremely complicated. Therefore, there is a problem that the manufacturing cost of the radius end mill becomes to be expensive.
With the popular radius end mill having cutting edges formed into complicated shapes and which is used for the high feed-speed cutting, essential cutting edges are formed at a distal end of the radius end mill If the cutting edges are worn, the cutting edges can not be re-ground for long-term use, and thus it is disposed of after use. Therefore, the life span of the radius end mill is short, and the term of the replacement cycle is short. As a result, there is a problem that the operating cost becomes expensive.
The present invention has been conceived in order to solve the above described problem, and it is an object thereof to provide a radius end mill and a cutting method using the radius end mill When a pocket is formed in the material, the radius end mill and the cutting method of the present invention enable the high feed-speed cutting at angle parts and an outline part of the pocket to efficiently cut a material. Further, the radius end mill and the cutting method enable the material's surface to be made smooth. Furthermore, the radius end mill and the cutting method are inexpensively manufacturable. In addition, the radius end mill and the cutting method enable the cutting edges to be re-ground for long-term use.